powerpc/mce: fix off by one errors in mce event handling

Before 69111bac42f5 ("powerpc: Replace __get_cpu_var uses"), in
save_mce_event, index got the value of mce_nest_count, and
mce_nest_count was incremented *after* index was set.

However, that patch changed the behaviour so that mce_nest count was
incremented *before* setting index.

This causes an off-by-one error, as get_mce_event sets index as
mce_nest_count - 1 before reading mce_event.  Thus get_mce_event reads
bogus data, causing warnings like
"Machine Check Exception, Unknown event version 0 !"
and breaking MCEs handling.

Restore the old behaviour and unbreak MCE handling by subtracting one
from the newly incremented value.

The same broken change occured in machine_check_queue_event (which set
a queue read by machine_check_process_queued_event).  Fix that too,
unbreaking printing of MCE information.

Fixes: 69111bac42f5 ("powerpc: Replace __get_cpu_var uses")
CC: stable@vger.kernel.org
CC: Mahesh Salgaonkar <mahesh@linux.vnet.ibm.com>
CC: Christoph Lameter <cl@linux.com>
Signed-off-by: Daniel Axtens <dja@axtens.net>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

1 files changed, 2 insertions, 2 deletions

diff --git a/arch/powerpc/kernel/mce.c b/arch/powerpc/kernel/mce.c
index 15c99b649b04..b2eb4686bd8f 100644
--- a/arch/powerpc/kernel/mce.c
+++ b/arch/powerpc/kernel/mce.c
@@ -73,7 +73,7 @@ void save_mce_event(struct pt_regs *regs, long handled,
                     uint64_t nip, uint64_t addr)
 {
         uint64_t srr1;
-        int index = __this_cpu_inc_return(mce_nest_count);
+        int index = __this_cpu_inc_return(mce_nest_count) - 1;
         struct machine_check_event *mce = this_cpu_ptr(&mce_event[index]);
 
         /*
@@ -184,7 +184,7 @@ void machine_check_queue_event(void)
         if (!get_mce_event(&evt, MCE_EVENT_RELEASE))
                 return;
 
-        index = __this_cpu_inc_return(mce_queue_count);
+        index = __this_cpu_inc_return(mce_queue_count) - 1;
         /* If queue is full, just return for now. */
         if (index >= MAX_MC_EVT) {
                 __this_cpu_dec(mce_queue_count);